Photons and quanta
Light delivers its energy in discrete packets called photons, each carrying an energy proportional to the light's frequency.
Practise this conceptLight delivers its energy in discrete packets called photons rather than as a smooth, continuous stream, and each photon's energy depends only on the light's frequency, not on how bright the light is.
That last point surprises people, because brightness and energy feel linked in everyday experience. Turning up the brightness of a light source doesn't make each individual photon carry more energy — it increases the number of photons arriving per second, while each one still carries exactly the same energy set by the light's colour.
This packet-based picture, proposed by Einstein in 1905, was a radical break from treating light purely as a wave, and it's the idea that eventually explains the photoelectric effect: whether a photon can knock an electron free depends entirely on that one photon's individual energy, not on the combined brightness of a whole beam.
Key ideas
Requires: Wave properties: amplitude, wavelength and frequency
Common misconceptions
- A photon's energy depends on the intensity (brightness) of the light, not its frequency.
- Photons are tiny particles with mass, just like electrons.
- Increasing the frequency of light increases the number of photons per second, not their individual energy.